Method for non-damaging charge injection and a system thereof

ABSTRACT

A method and system for injecting charge includes providing a first material on a second material and injecting charge into the first material to trap charge at an interface between the first and second materials. The thickness of the first material is greater than a penetration depth of the injected charge in the first material.

The present invention claims the benefit of U.S. Provisional PatentApplication Ser. No. 60/498,827, filed Aug. 29, 2003, which is herebyincorporated by reference in its entirety.

The subject invention was made with government support (InfotonicsTechnology Center (DOE)) Award No. DEFG02-02ER63410.A100. The U.S.Government may have certain rights.

FIELD OF THE INVENTION

The present invention generally relates to charge injection and, moreparticularly, relates to a method for non-damaging charge injection anda system thereof.

BACKGROUND

Recently, a new class of Micro-Electrical-Mechanical Systems (MEMS)devices have been disclosed that utilize embedded electronic charge as ameans for actuation or self-generating sensors, such as those disclosedin U.S. Pat. Nos. 6,597,560; 6,638,627; 6,688,179; 6,717,488; 6,750,590;and 6,773,488 and in US Patent Application Publication Nos.:2002/0131228; 2002/0182091; 2003/0079543; 2003/0201784; and 2004/0023236by way of example. Typically, charge is injected into the interface ofdissimilar insulating materials by high electric field injection.Electrons “e-” are caused to tunnel into the conduction band of amaterial, such as silicon dioxide, from a silicon substrate via ahigh-applied electric field. The electrons “e-” become trapped atelectronic trap sites at a composite insulator interface, such as aninterface between silicon dioxide and silicon nitride. The chargeremains trapped for an extremely long period of time and is thereforeuseful as MEMS enabling technology.

Embedded electronic charge is also useful for other macroscopicapplications, such as, but not limited to, harvesting energy from theenvironment. These macroscopic structures include windmills that canconvert the energy of wind into electrical power. However, formacroscopic structures it is impractical to embed electronic charge bytunneling into the conduction band of one member of insulating compositelarge structures. Typically, a suitable injecting interface, such assilicon to silicon dioxide, is not available.

One technique that has been investigated is to expose the structure to abeam of energetic particles such as an electron beam. With thistechnique, electrons “e-” impinge upon a surface of a compositeinsulating structure with sufficient energy to enter the system. Theseelectrons “e-” are then trapped at trap sites at a dissimilar insulatorinterface.

Unfortunately, there is significant difficulty with this ballisticelectron injection process. It is important that the energy of thearriving electrons “e-” be either below or above the range of energieswhere secondary electron yield is greater than unity. If the energy iswithin the range of greater than unity, a net positive charge cansignificantly affect the result. For example, positive charge within theoutermost insulator layer, but close to the embedded electron chargewill tend to empty the traps via internal high field, thus neutralizingthe effective trapped charge. Furthermore, the simple presence ofopposite sign charge in the vicinity of the trapped charge will tend toneutralize the effectiveness of the trapped charge.

Referring to FIG. 1, a graph of a secondary electron yield of silicondioxide as a function of electron energy is shown. The data in the graphshows that the secondary electron yield is greater than unity from about30 eV to approximately 3,800 eV. It is obvious any acceleratingpotential less than 30 eV does not have sufficient energy tosubstantially enter the system. Therefore, one must use energies greaterthan about 3,800 eV.

It is also desirable to create a system where the charge is as close tothe surface as possible. As a result, the thickness of the outermostlayer must be thin. However, as described above, the acceleratingpotential must be kept above the critical value of about 3,800 eV. Witha thin outermost layer and the accelerating potential above about 3,800eV, the penetration of the electrons “e-” may be too great.

Referring to FIG. 2, a graph of a Monte Carlo simulation of electronpenetration into a composite 10 of a layer of silicon dioxide 12 on alayer of silicon nitride 14 on a layer of silicon dioxide 16 that is ona substrate 18 of silicon is illustrated. The layer of silicon dioxide12, the layer of silicon nitride 14, and the layer of silicon dioxide 16each have a thickness of about 100 nm. The thickness of the outermostlayer of silicon dioxide 12 is chosen so that the average penetrationdepth of the arriving electrons “e-” is at the interface between theoutermost layer of silicon dioxide 12 and the layer of silicon nitride14. Unfortunately, this ballistic charge injection technique has notbeen shown to be effective.

Referring to FIGS. 3A and 3B, the capacitance-voltage (C-V)characteristics before and after the ballistic injection into thecomposite 10 are shown. The tests were performed on the composite 10 ofa layer of silicon dioxide 12 on a layer of silicon nitride 14 on alayer of silicon dioxide 16 with the substrate 18 of n-type silicon witha liquid InGa top electrode. The ballistic injection parameters were 3KeV, 100 sec., and 3,000 μC/cm² dose.

As the graphs in FIGS. 3A and 3B show, there is severe degradation inthe post-injection characteristics of the composite 10. This is presumedto be due to morphological changes creating defects. These defectsapparently have a wide energy distribution and significant dipolemoment. Investigations have determined poor retention time of charge forthese test structures. Furthermore, the maximum-trapped charge densityfor these investigations is much less than that achieved using highfield tunneling. Since the accelerating potential was in the range ofsecondary electron yield greater than unity, a slight negative shift isobserved indicating the presence of positive charge.

SUMMARY OF THE INVENTION

A method for injecting charge in accordance with embodiments of thepresent invention includes providing a first material on a secondmaterial and injecting charge into the first material to trap charge atan interface between the first and second materials. The thickness ofthe first material is greater than a penetration depth of the injectedcharge in the first material.

A system for injecting charge in accordance with embodiments of thepresent invention includes a first material on a second material with aninterface between the first and second materials and a charge sourcepositioned to inject charge into the first material to trap charge at aninterface between the first and second materials. The thickness of thefirst material is greater than a penetration depth of the injectedcharge in the first material.

The present invention provides a system and method for injecting chargeto fill the electronic traps at an interface between materials withoutcausing deleterious effects on charge storing characteristics of thematerials. The resulting structure with the trapped charge at aninterface is particular useful for MEMS enabling technology, but can beused to inject charge in other types of devices.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph of secondary electron yield as a function ofaccelerating potential;

FIG. 2 is a graph of a Monte Carlo simulation of 5 KeV ballisticelectron injection into a composite insulator;

FIG. 3A is a graph of C-V characteristics in a composite insulatorbefore electron injection;

FIG. 3B is a graph of C-V characteristics in a composite insulator afterelectron injection; and

FIG. 4 is a system for non-damaging electron injection in accordancewith embodiments of the present invention.

DETAILED DESCRIPTION

A system 20 for non-damaging electron injection in accordance withembodiments of the present invention is illustrated in FIG. 4. Thesystem 20 includes a structure 22 comprising a layer of silicon dioxide(SiO₂) 24 on a layer of silicon nitride (Si₃N₄) 26, a conductingelectrode 28, a ballistic electron source 30, and a power source 32,although the system 20 can comprise other types and numbers ofcomponents arranged in other manners. The present invention provides anumber of advantages including providing a system 20 and method forinjecting charge to fill the electronic traps at an interface 25 betweenlayers 24 and 26 that does not cause deleterious effects on chargestoring characteristics of the interface 25 between the layers 24 and 26of the structure 22.

Referring more specifically to FIG. 4, the structure 22 comprises thelayer of silicon dioxide (SiO₂) 24 on the layer of silicon nitride(Si₃N₄) 26, although other types and numbers of dissimilar insulatinglayers which are arranged in other manners can be used. An interface 25is formed between the layer of silicon dioxide 24 and the layer ofsilicon nitride 26 at which the trapped charge is stored.

The thickness of the layer of silicon dioxide 24 from an outer surface27 of the layer 24 to the interface 25 is about 500 micron, although thelayer of silicon dioxide 24 could have other thicknesses. The thicknessof the layer of silicon dioxide 24 is greater than a penetration depthof electrons “e-” injected into the layer of silicon dioxide 24 from theballistic electron source 30. The layer of silicon nitride 26 has athickness of about 100 nm in this example, although the layer of siliconnitride 26 can have other thicknesses.

The conducting electrode 28 is placed on another surface 29 of the layerof silicon nitride 26, although other manners for coupling the layer ofsilicon nitride 26 to the conducting electrode 28 can be used. A varietyof different types of conducting materials can be used for theconducting electrode 28.

The ballistic electron source 30 is an electron flood gun for injectingelectrons “e-”, although other types of charge sources can be used andother types of charge can be injected. The ballistic electron source 30is positioned and used to inject electrons “e-” through surface 27 intothe layer of silicon dioxide 24. The energy of the electrons “e-” fromthe ballistic electron source 30 is above the energy where secondaryelectron yield is unity. In these embodiments, the level of energy isabove 3,800 eV and may be on the order of 5,000 eV to 500,000 eV,although other levels of energy can be used. The injected electrons “e-”quickly will thermalize to the conduction band minimum of the layer ofsilicon dioxide 24. Since the number of injected electrons “e-” issubstantially greater than the secondary electrons, a local negativespace charge 33 is established. This will likewise establish an electricfield between the space charge and the conducting electrode 28 that issubstantially in contact with the surface 29 of the layer of siliconnitride 26.

The power source 32 applies a potential difference between theconducting electrode 28 and the electron source 30 which establishes anaccelerating potential for the electrons “e-” or other charge beinginjected, although the power supply 32 can be coupled to provide powerin other manners. An application of positive bias to the conductingelectrode 28 by the power source 32 may enhance the electric fieldacross the layer of silicon dioxide 24 and the layer of silicon nitride26.

A method for non-damaging electron injection in accordance withembodiments of the present invention will now be described withreference to FIG. 4. The layer of silicon nitride 26 is deposited on tothe layer of silicon dioxide 24, although the layers 24 and 26 can beformed in other manners. The layer of silicon dioxide 24 has a thicknesswhich is greater than a penetration depth of electrons “e-” injectedinto the layer of silicon dioxide 24 from the ballistic electron source30. The thickness for the layer of silicon dioxide 24 is determined by aMonte Carlo simulation taking into account the acceleration potentialsupplied by the power source 32 between the conducting electrode 28 andthe electron source 30 and the materials properties of the layer beinginjected into, in this example the layer of silicon dioxide 24, althoughother techniques for determining the thickness of the layer so that itis greater than the penetration depth of the charge being injected canbe used. The layer of silicon dioxide 24 on the layer of silicon nitrideform the structure 22, although again the structure 22 can have othernumbers and types of layers.

Next, the conducting electrode 28 is placed on the surface 29 of thelayer of silicon nitride 26, although other manners for coupling thelayer of silicon nitride 26 to the conducting electrode 28 can be used.The power source 32 is coupled to the conducting electrode 28 and to theballistic electron source 30 to apply a potential difference between theconducting electrode 28 and the ballistic electron source 30 whichestablishes an accelerating potential for the charge, in theseembodiments the electrons “e-”. The applied accelerating potential is ata value where secondary electron yield is less than unity. The powersource 32 may also apply a positive bias to the conducting electrode 28,although other biasing arrangements can be used, such as having theconducting electrode 28 coupled to ground.

The ballistic electron source 30 emits electrons “e-” in an electronbeam towards the surface 27 of the layer of silicon dioxide 24, althoughother types of charge could be used. The electrons “e-” penetratethrough the surface 27 and into the layer of silicon dioxide 24, but notto the interface 25 because the thickness of the layer of silicondioxide 24 is greater than the substantial maximum penetration depth ofthe electrons “e-” being injected.

Electrons “e-” injected in the layer of silicon dioxide 24 from theballistic electron source 30 migrate toward the interface 25 between thelayer of silicon dioxide 24 and the layer of silicon nitride 26 becauseof an electric field from a space charge region 33 in the layer ofsilicon dioxide 24 to the conducting electrode 28. The layer of silicondioxide 24 is a wide band gap material with a band gap of approximately9 eV making it an excellent insulator. However, the layer of silicondioxide 24 is basically a contact limited insulator. An electron in theconduction band of the layer of silicon dioxide 24 is actuallyreasonably mobile, on the order of 1 to 10 cm² per volt-second. As aresult, the injected electrons “e-” quickly fill the traps at theinterface 25 and remain there. Any morphological damage layer of silicondioxide 24 is well away from the interface 25 between the layer ofsilicon dioxide 24 and the layer of silicon nitride 26 and will notdegrade the characteristics of the trapped electrons, such as retentiontime. The trapped charge, in these embodiments electrons “e-”, at theinterface 25 is monopole charge.

If desired, a layer of photo resist or other protective material can becoated over the layer of silicon nitride 26 and a portion of the layerof silicon dioxide 24 may be etched away where the injection of theelectrons “e-” caused damage. By way of example only, hydrofluoric acid,can be used to remove the damaged portion of the layer of the silicondioxide 24, if desired. The layer of photo resist is then removed andthe structure 22 is ready for use in applications.

Accordingly, as described above, the present invention provides a system20 and method for injecting charge to fill the electronic traps at aninterface 25 between layers 24 and 26 that does not cause deleteriouseffects on charge storing characteristics of the interface 25 betweenthe layers 24 and 26 of the structure 22.

Having thus described the basic concept of the invention, it will berather apparent to those skilled in the art that the foregoing detaileddisclosure is intended to be presented by way of example only, and isnot limiting. Various alterations, improvements, and modifications willoccur and are intended to those skilled in the art, though not expresslystated herein. These alterations, improvements, and modifications areintended to be suggested hereby, and are within the spirit and scope ofthe invention. Additionally, the recited order of processing elements orsequences, or the use of numbers, letters, or other designationstherefor, is not intended to limit the claimed processes to any orderexcept as may be specified in the claims. Accordingly, the invention islimited only by the following claims and equivalents thereto.

1. A method for injecting charge, the method comprising: providing afirst material on a second material; and injecting charge into the firstmaterial to trap charge at an interface between the first and secondmaterials, wherein a thickness of the first material is greater than apenetration depth of the injected charge in the first material, whereinthe first and the second materials are dissimilar insulators.
 2. Themethod as set forth in claim 1 further comprising: providing a conductoron the second material; providing a source for the injected charge whichis positioned to direct the injected charge towards at least a portionof the first material; and applying an accelerating potential across theconductor and the source for the injected charge.
 3. The method as setforth in claim 2 wherein the applied accelerating potential is at avalue where secondary electron yield is less than unity.
 4. The methodas set forth in claim 2 wherein the applied accelerating potential isabove 3800 eV.
 5. The method as set forth in claim 1 further comprisingdetermining the penetration depth of injected charge in the firstmaterial, wherein the providing further comprises depositing the firstmaterial on the second material to the thickness which is greater thanthe determined penetration depth.
 6. The method as set forth in claim 1further comprising removing a portion of the first material where theinjected charge has penetrated.
 7. The method as set forth in claim 6further comprising coating at least a portion of the second materialwith a protective layer during the removal of the portion of the firstmaterial.
 8. The method as set forth in claim 1 wherein the firstmaterial comprises SiO₂ and the second material comprises Si₃N₄.
 9. Themethod as set forth in claim 1 wherein the charge trapped at theinterface is monopole charge.